1. Field of the Invention
The present invention relates to a magnetic resonance imaging system or a magnetic resonance imaging apparatus which is used for the measurement of the temperature of a specific region in thermotherapy using a heating system.
2. Description of the Related Art
A magnetic resonance imaging apparatus is an apparatus which images the chemical and physical microscopic information of a substance or observes a chemical shift spectrum by using a phenomenon in which when a group of nuclei having a unique magnetic moment is placed in a uniform static field, they resonantly absorb the energy of an RF magnetic field that rotates at a specific frequency.
On the other hand, there is available a thermotherapy of selectively destroying tumor tissue by heating a morbid region to a predetermined temperature or more and maintaining the temperature using the difference in thermosensitivity between the tumor tissue and the normal tissue. The treatment result of this thermotherapy greatly depends on temperature management for a region of interest.
A magnetic resonance imaging apparatus can be used for the measurement of the temperature of a treatment region (morbid region) in the above thermotherapy. As a temperature measurement technique using a magnetic resonance imaging apparatus, a temperature distribution measurement method or the like is available, which uses the temperature dependence of a water proton chemical shift as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 4-055257, U.S. Pat. No. 5,378,987, and the like. In this method, the measurement accuracy is degraded by the influence of the magnetic field nonuniformity of a target region due to the movement or the like of an object to be examined. In order to improve this, as disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-55642, there has been proposed a method of correcting an error by using, as a reference value, the phase information of a region exhibiting no temperature change (e.g., a region distant from a temperature measurement target like that shown in FIG. 1A or a fat tissue region exhibiting little temperature dependence).
In a conventional temperature measurement method using a water proton chemical shift, however, a temperature change is calculated on the basis of the phase information of a magnetic resonance signal. For this reason, it is impossible to discriminate whether a measured phase difference is caused by a real temperature change or an error component due to a system variation/movement of an object to be examined.
According to the method of correcting an error by using, as a reference value, the phase information of a substance region exhibiting no temperature change, when the distribution of changes in magnetic field nonuniformity is relatively low order, changes in magnetic field nonuniformity near a target region can be canceled out. If, however, changes in magnetic field nonuniformity exhibit a high-order distribution, an error component due to this cannot be removed.
In addition, according to each of the conventional techniques, when an object to be examined makes an abrupt movement during temperature measurement, discontinuity is observed in the phase value of a magnetic resonance signal as shown in FIG. 1B. For this reason, as shown in FIG. 1C, a temperature change is calculated by using a phase distribution after a phase change as a new reference. As a consequence, the continuous temperature changes from the start of heating processing shown in FIG. 1C cumulatively contain errors due to body movement. Furthermore, if body movement which is not observed as discontinuity occurs, it is impossible to properly discriminate a temperature change due to heating processing from an error due to body movement.